Science: How Wild Salmon Differs from Farmed Salmon and How to Cook Salmon to the Right Temperature

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Most home cooks pay close attention to internal temperature when cooking a steak, where we know just 5 degrees means the difference between rare and medium-rare. But few people pay that much attention when dealing with fish. Which is too bad, because even fatty fish like salmon can go from tender and moist to chalky and dry in a flash. In the test kitchen we use an instant-read digital thermometer to tell when salmon is done and we’ve always preferred it cooked 125 degrees for the ideal balance of firm, yet silky, flesh. The majority of the salmon we cook is farmed Atlantic, but as we’ve cooked more wild species we started to wonder if 125 was maybe a bit too high.
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To find out, we bought multiples filets of the four most common species of wild Pacific salmon—king (also known as Chinook), sockeye, coho, and chum. We cooked samples of each to both 120 degrees and 125 degrees sous vide, or sealed in a plastic bag and cooked in a temperature-controlled water bath. We also did the same for samples of farmed Atlantic salmon. We then asked tasters, blind to the differences in internal temperature, to pick which sample had the best texture.
Everyone preferred the coho, sockeye, and chum samples cooked to 120 degrees and the farmed Atlantic cooked to 125 degrees. While a few folks preferred the king sample at 125 degrees, the majority preferred 120 degrees.
These results may sound surprising. After all, salmon is salmon right? Well, not exactly. It turns out that farmed Atlantic salmon differs in two important ways from the half dozen commercial wild varieties caught in the Pacific Ocean:
1. Due to their more sedentary life, the collagen protein in farmed Atlantic salmon contains less chemical cross-links than in wild varieties, which translates into softer flesh.
2. Farmed Atlantic salmon contains more fat than any wild variety and up to 4 times as much fat as the leanest species. Fat provides the perception of juiciness when cooked.
So, with naturally firmer flesh, and less fat to provide lubrication, wild salmon can have the texture of overcooked fish even at 125 degrees. By cooking wild salmon to just 120 degrees the muscle fibers contract less and stay moist and tender.
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